The editor is the same machinery that produced the 142 cases in the casebook, handed to you. You draw a floor plan, cast the suspects, choose who dies, attach the witness statements — and then the editor checks your case the way it checks ours: exhaustively, before you can call it finished. It runs in your browser, it is free, and it needs no account.
Start with the grid. Boards are square, from a four-by-four one-room-and-a-corridor affair up to an eleven-by-eleven building, and you divide them into rooms by painting cells. A room can be any shape as long as it holds together, and it carries a name — kitchen, library, boot room — which the clue texts then use. Rooms are what most of the interesting statements are about, so a plan with one enormous hall and three cupboards gives you far less to work with than a balanced one.
Then the people. A board of width six holds five suspects and one victim, because the first rule of the game is that every person occupies their own row and their own column. Each suspect gets a face: hair colour, a beard or none, glasses or none, a hat. Those visible features are what the witness statements refer to, so the cast you assemble decides which statements are even available to you. The victim shows only its gender — the rest is deliberately hidden, and no clue may depend on a feature the player cannot see.
The clue builder offers every statement type the game knows: which room somebody was in, which direction they stood from somebody else, how far apart two people were, whether a room was empty, whether anyone was beside the fireplace. Statements can be combined with and, or and not, and there are global statements that describe the whole board at once rather than one person — those are often what makes an opening possible.
Anyone can write a puzzle. The hard part is knowing whether it is a puzzle at all, and that is what the editor does for you. It runs two passes over your draft.
The first is an exhaustive search: it enumerates every legal arrangement of your cast on your floor plan and counts how many satisfy your statements. If the answer is zero you have contradicted yourself somewhere. If it is more than one, your case is ambiguous and the editor will say so rather than let you publish a puzzle with two answers.
The second pass is the one solvers care about. It solves your case the way a person does — forward only, one justified elimination at a time, never assuming a possibility to see where it leads. A case that passes the first check but fails this one has exactly one answer that can only be found by trial and error, which is bookkeeping rather than deduction. The editor tells you which of the two you have made.
You do not have to write every statement yourself. Place your people, describe as much as you want to, and ask the editor to complete the case: it will search for a set of statements that pins your arrangement down to a single forward-solvable answer, and discard any statement the case turns out not to need. That last part matters more than it sounds — a clue the solver reads, works with and then discovers was never required is the most common flaw in hand-written puzzles.
Three problems account for most drafts that will not resolve. The first is a floor plan with too few rooms: when every room borders every other room, a statement about which rooms are next to each other tells the solver nothing, and half the clue vocabulary stops working. Four to eight rooms on a mid-sized board is the range that behaves.
The second is writing statements that cannot mean anything. Because every person occupies their own row and their own column, no two people ever stand directly beside one another, share a row, or sit on a line between two others. A statement built on any of those is either impossible or true by default, and the builder will not offer you one — if a clue type you expected is missing, this is usually why.
The third is loading everything onto one suspect. A case where each statement pins one person independently is solvable but flat: the reader works through a checklist. The cases people remember are the ones where a statement about one suspect is what finally places a different one, and the fastest way to build that is to give your most-constrained person fewer clues, not more.
Your drafts and finished cases live in your own browser, on your own device; nothing is uploaded and there is nothing to sign into. Export a case to a file to keep it, to move it to another machine, or to send it to somebody else to solve. Import works the same way round. Anything you build can be played immediately in the normal game screen, hints and all.